1. Field of the Invention
The present invention relates to an adhesive for fuel cells and membrane-electrode assembly produced using the same.
2. Related Art
In recent years, fuel cells have been greatly anticipated to be a means for curbing global warming and environmental destruction, and additionally to be a next-generation system for electric power generation, and thus research and development have been extensively carried out. Fuel cells generate energy by an electrochemical reaction of hydrogen and oxygen, and examples thereof include phosphoric acid fuel cells, molten carbonate fuel cells, solid electrolyte fuel cells, solid polymer fuel cells, and the like. Among these, solid polymer fuel cells have drawn attention as power sources for automobiles (two wheeled, four wheeled, etc.), portable electric power sources and the like, since they can be driven even at ordinary temperatures and have high output powers and compact size.
In solid polymer fuel cells, a fuel gas supplied on an anode side electrode, for example, a gas containing mainly hydrogen (hereinafter, also referred to as “hydrogen-containing gas”) moves to a cathode side electrode side through a solid polymer electrolyte membrane after hydrogen is ionized on the electrode catalyst. Electrons generated during this process are taken out into an external circuit, and utilized as electric energy in DC. It should be noted that, since oxidizer gas, for example, air or a gas mainly containing oxygen (hereinafter, also referred to as “oxygen-containing gas”) is supplied to the cathode side electrode, hydrogen ions, electrons and oxygen react on the cathode side electrode, whereby water is generated.
According to the membrane-electrode assembly in the aforementioned solid polymer fuel cells, a configuration in which a solid polymer electrolyte membrane, and an anode side electrode and a cathode side electrode are provided to have the same outside dimensions (hereinafter, referred to as “first configuration”), as well as a configuration in which the aforementioned solid polymer electrolyte membrane has an outside dimension greater than the outside dimensions of the anode side electrode and the cathode side electrode (hereinafter, referred to as “second configuration”) have been employed.
Incidentally, in attempts to reduce the overall size of the fuel cell described above, decreasing the membrane thickness of the solid polymer electrolyte membrane that configures the membrane-electrode assembly has been desired. However, in the aforementioned first configuration, the end face position of the solid polymer electrolyte membrane matches with the end face positions of the anode side electrode and the cathode side electrode, and thus the fuel gas supplied to the anode side electrode and the oxidizer gas supplied to the cathode side electrode may indirectly go around from the end face of the solid polymer electrolyte membrane, which may result in mixing. In addition, a problem has been indicated in that short circuiting may be likely to occur between the end faces of the anode side electrode and the cathode side electrode. Furthermore, in the second configuration described above, the strength of a portion of the solid polymer electrolyte membrane sticking out from the end faces of the anode side electrode and the cathode side electrode is likely to be lowered, and thus, there arises a problem in that the solid polymer electrolyte membrane is more easily broken.
Consequently, the membrane-electrode assembly disclosed in Patent Document 1 has been known, for example. In Patent Document 1, a gas diffusion electrode layer 2 is provided on one surface of a solid polymer electrolyte membrane 1 to cover this surface, and on another surface of the solid polymer electrolyte membrane 1 is provided a gas diffusion electrode layer 3 having a surface area smaller than that of this other surface, as shown in FIG. 5. The gas diffusion electrode layers 2 and 3 have catalyst layers 4a and 4b that are in contact with two surfaces of the solid polymer electrolyte membrane 1, and gas diffusion layers 5a and 5b. The catalyst layers 4a and 4b are designed to have different dimensions from one another. An adhesion layer 6 is provided at a periphery of the catalyst layer 4a, and the gas diffusion electrode layer 2 is integrated with the solid polymer electrolyte membrane 1.
Moreover, the fuel cell disclosed in Patent Document 2 has been known as a solid polymer fuel cell having an adhesion layer. The fuel cell disclosed in Patent Document 2 is characterized by having a mixed layer in which a catalyst layer and an adhesion layer are present together on locations along the circumference of the catalyst layer. According to the fuel cell disclosed in Patent Document 2, the solid polymer electrolyte membrane is prevented from being damaged by avoiding formation of a distinct boundary between the catalyst layer and the adhesion layer.
In addition, an exemplary adhesive for fuel cells studied with regard to use in producing fuel cells is disclosed in Patent Document 3. When the adhesive for fuel cells disclosed in Patent Document 3 is used, curing can reportedly be ensured, irrespective of the adherend. However, bond durability is not satisfactory, and further improvement has been demanded.